Please note this question is about Azure Active Directory Domain Services - https://azure.microsoft.com/en-gb/documentation/services/active-directory-ds/
Not the similarly named Azure Active Directory - https://azure.microsoft.com/en-gb/services/active-directory/
If I have an Azure site-to-site VPN to the vnet on which Azure Active Directory Domain Services is installed can I join an on premises server or client to the Azure Active Directory Domain Services domain.
The simple answer is Yes.
Few limitations comes to my mind: